The present invention relates to an apparatus for producing nano-sized metal or ceramic powders, and more particularly, it relates to an apparatus for preparing a composite of metals and ceramics at a high production rate.
Nano-scale particles are known to exhibit unique physical and chemical properties. The novel properties of nano-crystalline materials are the result of their small residual pore sizes, limited grain sizes, phase or domain dimensions, and large fraction of atoms residing in interfaces. In multi-phase materials, limited phase dimensions could imply a limited crack propagation path if the brittle phase is surrounded by ductile phases, so the cracks in a brittle phase would not easily reach a critical crack size. Even with only one constituent phase, nano-crystalline materials may be considered as two-phase materials. The possibilities for reacting, coating, and mixing various types of nano-materials create the potential for fabricating new composites with nano-sized phases and novel properties. Not only is the structure different from those exhibited by their bulk counterparts, but also the mechanical, electronic, optical, magnetic and thermal properties of nano-crystalline materials are different from those exhibited by their bulk counterparts. Specifically, ceramics fabricated from ultra-fine particles are known to possess high strength and toughness because of their ultra-fine intrinsic defect sizes and the ability for their grain boundaries to undergo a large plastic deformation. Ultra-fine particles can be sintered at much lower temperatures also.
Known techniques for generating nanosized particles may be divided into four broad techniques, including: a vacuum technique, a gas-phase technique, a condensed-phase synthesis technique, and a mechanical grinding technique. The vacuum synthesis techniques includes sputtering, laser ablation, and liquid-metal ion source. Additionally, the gas-phase technique includes inert gas condensation, oven sources (for direct evaporation into a gas to produce an aerosol or smoke of clusters), laser-induced vaporization, laser pyrolysis, and flame hydrolysis. Furthermore, the condensed-phase synthesis technique includes reduction of metal ions in an acidic aqueous solution, liquid phase precipitation of semiconductor clusters, and decomposition-precipitation of ionic materials for ceramic clusters. Moreover, the mechanical grinding technique deals with a ball mill by which the powders in the mill pots can be ground into ultra-fine particles. Regardless of the technique used, most of these prior-art techniques suffer from a severe drawback: extremely low production rates. It is not unusual to find a production rate of several grams a day in a laboratory scale device. Vacuum sputtering, for instance, only produces small amounts of particles at a time. Laser ablation and laser-assisted chemical vapor deposition techniques are also well-known to be excessively slow processes. These low production rates, which generally result in high product costs, have severely limited the utility value of nano-phase materials. There is, therefore, a clear need for a faster, more cost-effective method for preparing nano-sized powder materials. Some processes require expensive precursor materials to produce ceramic powders and could result in a harmful gas. Most of the prior-art processes are capable of producing one particular type of metallic or ceramic powder at a time, but do not permit the preparation of a uniform mixture of two or more types of nano-scaled powders at a predetermined proportion. Also, most of the prior art processes require heavy and/or expensive equipment, resulting in high production costs. Additionally, during the precipitation of ultra-fine particles from the vapor phase, when using thermal plasmas or laser beams as energy sources, the particle sizes and size distribution can not precisely be controlled. Also, the reaction conditions usually lead to a broad particle size distribution as well as the appearance of individual particles having diameters that are multiples of the average particle size.
Prior art disclosures about mechanical grinding methods are as follows: U.S. Pat. No. 3,524,735 (Aug. 18, 1970), U.S. Pat. No. 5,356,084 (Oct. 18, 1994) and 5,375,783 (Dec. 27, 1994) by Rodger. L. Gamblin. U.S. Pat. No. 5,035,131 (Jul. 30, 1991), 5,113,623 (May 19, 1992), 5,170,652 (Dec. 15, 1992), 5,187,965 (Feb. 23, 1993), 5,287,714 (Feb. 22, 1994), Canadian Patent 2024120 (Aug. 28, 1990), 2044658 (Jul. 14, 1991) by Dieter Figge and Peter Fink. The U.S. Pat. No. 3,524,735 is similar to present invention in all these prior arts, but it""s structure is simple and its producing efficiency is very low. Usually it needs more than 10 hours for preparing only several hundred grams of nanosized powders. For a review on this topic, two papers are refered: Chen, Shizhu, xe2x80x9cResearch on Working Principle of a Planetary High-energy Ball Millxe2x80x9d Mining and Metallurgical Engineer, V17 n4 December 1997. C. C. Koch, xe2x80x9cThe Synthesis and Structure of Nanocrystalline Materials Produced by Mechanical Attrition: A Reviewxe2x80x9d NanoSTRUCTURE MATERIALS Vol. 2, pp. 109-129, 1993.
FIG. 1 is the schematic of the prior art U.S. Pat. No. 3,524,735. A small drive rotating pulley 2 is connected to a motor 1 and receives rotational forces therefrom. These rotational forces are transmitted from a small pulley 2 to a large pulley 4 through a belt 3. Mill pots 7 are held symmetrically on a rotary turntable 5. This rotary turntable, also referred to as the main shaft, is mounted on the same rotary shaft as the large pulley 4. The central rotary shaft 6 of each mill pot 7 forms a revolving pair with the turntable 5. The bottom end of the shaft 6 is connected to and supported by a planetary pulley 9. The pulley 9 corresponding to each mill pot is connected to a central pulley 8 through a belt based transmission system, forming a planetary motion pair. The central pulley 8 is disposed coaxially with the large drive pulley 4 and the turntable 5 on the same base. The two drive pulleys 4, 5 and the auxiliary central pulley 8 share a common central axis. When starting the motor 1, the turntable 5 will rotate and all the mill pots will undergo a primary revolving motion surrounding the central axis, at the same time, each mill pot 7,working congruently with the auxiliary pulley 8, will make a planetary motion. In this vertical style ball mill, each mill pot 7 not only revolves about an axis of the main shaft, but also revolves around its own axis. The producing efficiency of this conventional high-energy planetary ball mill is very low. Additional problems exist with respect to this conventional ball mill, including the following: First, the pivot shaft to support mill pot is revolvable and cantalevered, so the root of the pivot shaft bears the majority of the stress, which may result in a tire rupture of the pivot shaft and limit the rotating speed of this ball mill. Second, due to the mill pots being positioned upright, the powdered particles will deposit on the bottom of the mill pots and substantially prevent the particles from being fined continuously.
If the power and efficiency of the ball mill can be significantly improved, mechanical grinding methods can become a mass production method for preparing nano-scaled powders. The present invention is a reasonable high energy planetary ball mill (see FIG. 2, FIG. 3, FIG. 4 and FIG. 5).
A preferred embodiment of the present invention is a planetary high-energy ball mill for producing nanometer-scale powders. This ball mill is composed of the following major components: a vertical main shaft that is revolvable and glide-able up or down, a turntable fixed on the top of a main shaft, a plurality of mill pots fixed inside the cup-like rollers, wherein the rollers are rotatably supported by the swing-able pivotal shaft, a stationary ring which is disposed coaxially with the main shaft, a transmission screw for driving a clamp, and two motors for driving the main shaft and transmission screw, respectively.
When the main shaft is rotated by the main motor via the gear pair, the turntable will rotate too, and the cup-like rollers will be pushed to touch the stationary ring due to the centrifugal force. The cup-like rollers can also rotate about their own axis due to the friction counterforce received from the stationary ring. In this invention, a cup-like roller is not only revolvable around its own axis, but also revolvable around the main shaft, such that the mill pots fixed inside the cup-like rollers are in emblematical planetary motion. Because this planetary movement of the mill pots, the particles in the mill pots can be ground to ultra-fine powders. These fine powders will deposit on the bottom of the mill pots after the ball mill works several hours. This is a general problem for the ball mill with upright mill pots, wherein the thick deposited powders will stymie the powders in the mill pots to be fined sequentially. In this invention, after the ball mill works for several hours, the main shaft will be operated to rotate slowly, and a screw driving means will drive the main shaft up until the cup-like rollers approach a horizontal position, then each cup-like roller, as well as the mill pot, will rotate on the top of the stationary ring. Several magnetizer impacting poles, which are inserted into a corresponding hole of the cup-like roller, may be attracted to or repelled from a magnet fixed under the turntable periodically. When a magnetizer impacting pole is near the magnet, a spring will be compressed, alternatively, when a magnetizer impacting pole is away from the magnet, the magnetizer impacting pole will impact the bottom of the mill pot by the elasticity of the spring. It is the frequent impact that disperses the deposited powders. After the deposited powders are dispersed, the main shaft will be operated to move down till the axis of the mill pots are upright, and then the ball mill will be operated to rotate in faster speed. It is possible to repeat above up and down process several times, the powders in the mill pots will be ground into nanoscale, and further, the mechanical alloying or the composite will be realized.
Advantages of the present invention may be summarized as follows:
(1) In present invention, first, each swing-able pivotal shaft by which the cup-like roller is rotatably supported is a two-points supported beam in mechanics, so its stress distributing can be improved compared to the prior art in which the pivotal shaft is a cantalever beam. Second, because the swing-able pivotal shaft can not rotate about its own axis, the tire rupture phenomenon of the swing-able pivotal shaft will be avoided. Therefore the present invention can be operated to rotate faster, resulting in the powders in the mill pots being crushed by the balls at higher frequency and stronger crushing force than prior art. Because of the above-mentioned benefits, the present invention can be used to produce nanosized powders at industrial level.
(2) In present invention, the powders deposited on the bottom of the mill pots can be dispersed by the impacting poles when the mill pots lay at horizontal position. This is propitious to fine the powdered particles continuously till nano-scale.
(3) A variety of nano-scaled ceramic, metal and mechanical alloying or composite which are comprised of ceramics and metals, can be readily produced by using present invention. The metal materials can be selected from the period 1c table, and the ceramic materials can be selected from the group consisting of oxide, carbide, nitride, chloride, boride, silicide, sulfite and so on.